Get yourself invited to the w3c GitHub organization, and accept the invite. You can make your membership public if you want. This (plus the account linking above) should mean you’re automatically added to
https://github.com/orgs/w3c/teams/w3c-group-32061-members (where you can check whether you're in the groups). The invite should be doable by any
W3C staff. (TODO: Is this step still needed?)